Ion mobility spectrometry is used in chemical and biological agent detectors and provides good sensitivity, low power requirements, and operation at atmospheric conditions. In general, ion mobility spectrometry is a gas-phase ion separation technique that separates different chemical species as a function of both size, i.e. average collisional cross section area, and mass-to-charge ratio (m/z). The mass-to-charge ratio is the molecular weight of a species divided by the number of charges, which in many instances is one because that species is singly charged. In a typical ion mobility spectrometry device, a sample to be analyzed is collected and passed through an inlet and into an ion source region where ions are formed. The ions then pass along a drift tube containing a potential gradient that is used to accelerate the ions against a counter-current drift gas, e.g. air. Air is also used as a carrier gas to urge the ions into the drift tube. Under the influence of the accelerating voltage, the lighter and smaller ions, i.e. the ions having a smaller mass-to-charge ratio and average collisional cross section area, reach the detector first, and heavier and larger ions arrive later.
As the ions exit the drift tube, they collide with a detector or collector, for example a Faraday Cup. Since the ions exit the drift tube at different times, chemical species in the sample are identified based on known arrival times of certain ions at the detector. When a given ion or group of ions reach the detector, they create a voltage peak that is proportional to the number of ions striking the detector. These peaks are referred to as the ion mobility spectrometry spectra. The ion mobility spectrometry spectra are averaged to increase the signal-to-noise ratio (SNR) for a given measurement. Therefore, a time window for monitoring the detector is established and monitored for these peaks. In addition, a voltage threshold is established for each peak, and the number of peaks in excess of the voltage threshold is monitored. An alarm condition in the monitoring device is established for a given contaminant in the sample when a sufficient number of peaks above the voltage threshold that are associated with that contaminant are detected.
Ion mobility spectrometry begins by forming reactant ions through the interaction of reactant molecules with a radioactive source. Typically, sample ions are formed in the ionization region by collisions between reactant ions and sample molecules. This occurs in the gas phase by electron- or proton-transfer from reactant ions available in the ionization chamber to sample molecules, i.e. atmospheric pressure chemical ionization (APCI). A sweep gas is often used to urge the sample ions toward the drift tube. In addition, once the sample ions are admitted into the drift tube, they are exposed to both a potential gradient and a counter current drift gas. Since the analysis takes place at atmospheric conditions, air is often used as both the sweep gas and counter current drift gas which can contain varying amounts of water vapor.
Formation of ion clusters is a common problem in ion mobility spectrometry. The existence of water vapor in the sample flow increases the problem of ion clusters. Therefore, the sweep gas and counter current gas need to be as dry as possible. In the laboratory, drying of these gases can be facilitated by any process desired regardless of size, cost or complexity. When these analyzers are used in the field, however, size and cost are considerable concerns.
In field analyzers, the sample air is typically re-circulated in a closed-loop type system. Molecular sieves and charcoal filters are included in these closed-loop systems to remove the water vapor and organic compounds from the sample air. The molecular sieve and charcoal need to be changed, cleaned, replaced or re-activated regularly. In one system, for example, the molecular sieve and charcoal were provided as a removable cartridge that has to be replaced every 3-10 days depending on use, creating a need to monitor the system to determine when the cartridge needs to be replaced and adding an additional cost associated with the consumable cartridges.
Therefore, a system is needed for removing moisture or water vapor from the sample gases in an ion mobility spectrometry analyzer that can operate continuously and that eliminates the need for consumables.